It is well known that heat destroys microorganisms. The presence of moisture accelerates this destruction by denaturing or coagulating the proteins making up the microorganisms. Most microorganisms contain sufficient water so that moderate heat alone, e.g. 80° C.-100° C., will destroy the microorganism. Many bacterial spores, on the other hand, contain substantially no water and require elevated temperatures, in excess of 150° C., for their destruction where dry heat is used. Hence, the destruction of such organisms is generally carried out in the presence of steam in autoclaves.
Chemical indicators have been developed to indicate whether an acceptable level of sterilization was achieved in a sterilization cycle. One such product is known as Temp-Tube, and is disclosed in, for example, Kelson, U.S. Pat. No. 3,313,266, White, U.S. Pat. No. 3,341,238, and White, U.S. Pat. No. 3,652,249. The device consists of a sealed tube containing a compound with a melting point which corresponds to the sterilization temperature. The device is capable of indicating whether or not the autoclave was held at a temperature above or below the melting point for a period of time once the melting point is reached.
Other sterility indicators are known. One such indicator is disclosed in Larsson, U.S. Pat. No. 3,981,683, and uses a backing strip of aluminum foil having an organic compound containing oxygen or nitrogen in contact with a wicking strip, and a cover strip overlying the organic compound and the wicking strip. The cover strip is a polymeric rate controlling film that permits water vapor to pass through at a rate sufficient to make the strip operable at a temperature to be monitored. Foley, U.S. Pat. No. 4,448,548 also discloses a steam sterilization indicator.
Further, Class 5/Type 5 sterilization indicators, which are configured to react to three critical sterilization parameters, i.e. time, temperature and the presence of steam, of sterilization cycles are available. Class 5/Type 5 indicators are configured such that their performance correlates to a biological indicator (BI). A Class 5/Type 5 indicator may be configured to work for more than one sterilization condition. For example, a Class 5/Type 5 indicator may be configured for a steam sterilization cycle at 121° C. for 30 minutes or a steam sterilization cycle at 132° C. for 4 minutes.
Class 6 sterilization indicators, which are configured to react to the three critical sterilization parameters for a specified sterilization cycle, are also available. U.S. Pat. No. 9,623,134, which is assigned to the Applicant of the present application and incorporated herein by reference, discloses sterilization test strips configured to indicate whether an acceptable level of steam sterilization has occurred after a predetermined period of time at a predetermined temperature. For example, a test strip may be configured to indicate whether an acceptable level of steam sterilization has occurred after 4 minutes at 134° C. However, these test strips are configured to work for one specific steam sterilization condition (e.g. 4 minutes at 134° C.) Thus, a different test strip is required for each different sterilization condition. For example, the test strip configured for a steam sterilization cycle at 132° C. for 4 minutes will not properly indicate for a steam sterilization cycle at 121° C. 12 minutes. As such, users who perform sterilization processes in multiple different conditions are forced to purchase and stock multiple different test strips, which can increase operation costs and lead to user errors in selecting a correct indicator.
U.S. Pat. No. 9,354,227, which is also assigned to the Applicant of the present application and incorporated herein by reference, discloses a dual sterilization indicator configured to work for two different sterilization conditions. The dual indicator includes two pass zones, each of which is configured for indicating acceptability of a sterilization cycle at a different sterilization condition. As such, the dual indicator may replace two different Class 6 sterilization indicators, and thus, can reduce a number of different test indicators required for users.
The dual indicator is configured for two different sterilization conditions. Further, Class 5/Type 5 indicators may be used for more than one sterilization conditions. Thus, these indicators may present a risk of confusion in determining the sterilization condition used for a steam sterilization cycle. Therefore, there is a need for an improved indicator that can provide an indication of sterilization conditions used in a sterilization cycle along with indication of acceptability of the sterilization cycle.